Chitin, the second most abundant natural biopolymer (after cellulose), is a significant structural component in the shells of crustaceans (e.g., crabs, lobster and shrimp), in the exoskeletons of insects and in the cell walls of many microbes and higher fungi. Chitin is a polysaccharide consisting predominantly of unbranched chains of β-(1,4)-2-acetamido-2-deoxy-D-glucose (also known as N-acetyl-D-glucosamine) residues. It may also be regarded as a derivative of cellulose, in which the C-2 hydroxyl groups have been replaced by acetamide residues, and it resembles cellulose in many of its properties. Its occurrence in nature and its isolation are well documented.
Chitin is an amorphous solid which is practically insoluble in water, dilute acids, dilute and concentrated alkalies, alcohol and other organic solvents. It is soluble in concentrated HCl, H2SO4, 78-97% H3PO4, and anhydrous HCOOH.
In the U.S. and most other countries, chitin is a greatly underutilized resource and even a significant waste problem for the shellfish industries. The amount of chitin potentially available from seafood wastes in the U.S. was estimated at between 5000 to 8000 tons per year by Hattis and Murray (Industrial Prospects for Chitin From Seafood Wastes, MIT Seagrant Report No. 27, MIT, Cambridge, Mass., August 1976).
Chitin is obtained or isolated from chitinaceous materials such as the shells of crustaceans by removing the associated minerals, principally calcium carbonate, and proteins from the chitinaceous material. The minerals are typically removed by reacting the chitinaceous material with an acid, generally hydrochloric acid, which produces a water soluble chloride by-product. The proteins are typically removed by reacting the chitinaceous material with a base, generally sodium hydroxide.
It has been recognized that chitosan formed by deacetylating chitin has interesting and potentially useful properties. Although chitins may occur in nature in a slightly de-acetylated form, that which has been purposely de-acetylated is usually called chitosan.
Chitosan is not a single, definite chemical entity, but varies in composition depending on conditions of manufacture. It may be equally defined as chitin sufficiently deacetylated to form soluble amine salts.
Solutions of chitosan may be highly viscous, resembling those of natural gums. The cationic properties of the polymer lead to formation of complexes with anionic polyelectrolytes such as carboxymethyl cellulose, and the reactivity of the amino group permits formation of stable gels with a variety of cross-linking agents. Many potential uses for chitosan have been developed, including flocculating agents for water and waste treatment, an additive for drilling fluids, a chelating agent for removal of traces of heavy metals from aqueous solutions, coating to improve dyeing characteristics of glass fibers, wet strength additives for paper, adhesives, photographic and printing applications, thickeners, formation of fibers and films, and many others. Other uses and processes are described in U.S. Pat. Nos. 3,862,122; 3,922,260; 4,018,678; 4,195,175; 5,010,181; and 6,310,188. Commercial development of chitosan has, however, been hampered by the cost of manufacture.
Chitosan was first described by E. Gilson (Berichte 28 821 (1895); Bull (3) 11 1099 (1894)) as prepared by heating chitin with concentrated potassium hydroxide at 180° C., or by fusion with solid potassium hydroxide. Later investigators, notably Rigby (U.S. Pat. No. 2,040,879, May 1, 1936), used alkali hydroxide, usually sodium hydroxide in concentrations from 30 to 60 percent by weight and temperatures from 80° to 160° C., to produce deacetylated chitin products ranging from 20 percent to complete deacetylation. In general, reaction times necessary to obtain soluble products, i.e., chitosan, were found to vary inversely with alkali concentration and temperature, but no consistent correlation of these variables seems to have been developed. In all cases, the ratio of sodium hydroxide solution to chitin used in deacetylation has been high, amounting to three or more parts of alkali hydroxide on a 100 percent basis per part of chitin. Agitation of the chitin-alkali mixture during reaction has also been considered necessary to obtain uniformity of product, and exclusion of air has been found necessary to minimize degradation.
The combination of high alkali concentration, high ratios of alkali solution to chitin treated, high reaction temperature, and agitation during reaction results in high cost for the conversion of chitin to chitosan. High temperature and high alkali concentration require corrosion resistant apparatus, generally all nickel or nickel lined, adding greatly to equipment costs. High ratios of alkali liquid to chitin increase equipment sizes for equal chitosan production and added chemical costs.
U.S. Pat. No. 4,195,175 discloses the process for the deacetylation of ground chitin to yield a soluble chitosan product, that comprises kneading the ground chitin with substantially 2 to 7 parts of substantially 35 to 50 percent sodium hydroxide solution, heating the resulting mixture to substantially 40° C. to 80° C., packing the mixture in containers to expel entrapped air, sealing the containers and displacing residual air with nitrogen, holding the mixture in a quiescent state at substantially 40° C. to 80° C. for from substantially 160 to 40 hours, removing the alkali solution, and washing and drying the resulting chitosan product.
U.S. Pat. No. 4,619,995 discloses that chitosan can be prepared by (a) dispersing chitin in a liquid medium of the group consisting of isopropyl alcohol, n-butanol, isobutanol, methyl ethyl ketone, toluene, and ethanol-toluene mixtures containing at least 72 mole percent toluene, (b) slowly adding a strong aqueous sodium hydroxide solution to the stirred slurry over a period of 10 to 30 minutes and in a proportion to provide substantially five to nine mols sodium hydroxide per mole N-acetyl glucosamine units in the chitin (about 0.98 to about 1.75 g sodium hydroxide per g chitin), (c) heating the stirred slurry to a temperature in the range of 75° C. to 100° C. and maintaining this range for a period of 2.5 to 3.5 hours, and (d) allowing the slurry to cool to ambient room temperature and to steep the chitosan in the caustic medium for a brief period, typically 0.5 to 1.0 hours. The liquid medium is chosen such that the chitosan produced is swollen by the sodium hydroxide.
U.S. Pat. No. 4,574,150 discloses a process for the manufacture of a dry, free-flowing, water soluble carboxylic acid complex of chitosan which includes the step of combining one part of chitosan with from 0.5 to about 30 parts by weight of a liquid selected from the groups consisting of alkanes containing about five to about nine carbon atoms, monoketones, monoesters, mono- and di-ethers, mononitriles, mononitroalkanes containing two to four carbon atoms, normally liquid mono-, poly-chloroalkanes, and alkenes containing one to two carbon atoms.